In recent years, it is possible for DVD (digital versatile disc) spreading promptly as an optical recording medium for image information to record information of 4.7 GB per one surface by using a red semiconductor laser with wavelength of 650 nm and an objective lens with numerical aperture (NA) of 0.6. However, there is a strong demand for higher density and greater capacity, for conducting recording/reproducing of higher density information at higher transfer rate. To attain higher density and greater capacity of the optical disc, it is possible to reduce a diameter of a spot obtained through light-converging by an objective lens, as is known widely, and for that purpose, it is necessary to make a wavelength of a light source to be shorter, and to make a numerical aperture of an objective lens to be higher.
With respect to a shorter wavelength of a laser light source, there have been put to practical use a violet semiconductor laser with wavelength 405 nm and a violet SHG laser, and a combination of the violet laser light source and an objective lens with NA 0.6 makes it possible to record information of about 15 GB per one surface (hereinafter, an optical disc employing a violet laser light source is called a “high density optical disc” as a general term, in the present specification).
With respect to higher NA of an objective lens, there has been proposed a standard of an optical disc for conducting recording/reproducing of information by converging a light flux emitted from a violet laser with an objective lens with NA of 0.85, and an optical disc of this standard makes it possible to record information of about 23 GB per one surface, for an optical disc whose diameter is 12 cm.
For the proper recording/reproducing of information for a high density optical disc, it is necessary to prevent deterioration of light-converging function caused by instantaneous wavelength changes of a laser light source that is called mode hopping, by providing a means to correct longitudinal chromatic aberration. The reason for the foregoing is that a focus error is generated to be large by even a slight change of wavelength, because wavelength dispersion of lens material in the violet area becomes extremely great. As an optical item for correcting longitudinal chromatic aberration which is suitable for cost reduction, weight reduction and downsizing, there is available a diffracting lens wherein a diffractive structure is formed on the surface of a refracting lens.
Only ability to conduct proper recording/reproducing of information for high density optical disc is not a sufficient value of an optical disc player product. When considering the present actual where DVD and CD (compact disc) on which various pieces of information are recorded are sold, only ability to conduct recording/reproducing of information for high density optical disc is not sufficient, and another ability to conduct proper recording/reproducing of information equally for DVD and CD owned by a user, for example, enhances a commercial value of an optical disc player for a high density optical disc. From the background of this kind, it is desired that an optical pickup device housed in an optical disc player for high density optical disc has power to conduct proper recording/reproducing of information while maintaining interchangeability for any of high density optical disc, DVD and CD.
As a method to conduct proper recording/reproducing of information while maintaining interchangeability for any of high density optical disc, DVD and CD, there is considered a method to switch optical parts for high density optical disc and optical parts for DVD and CD selectively, depending on recording density of the optical disc for recording/reproducing of information. However, this method is disadvantageous for downsizing, and the cost thereof is increased.
Therefore, for simplifying the structure of the optical pickup device and thereby for reducing the cost, it is preferable to standardize optical parts for high density optical disc and optical parts for DVD and CD to reduce the number of parts constituting the optical pickup device as far as possible, even in the optical pickup device having interchangeability.
Therefore, when arranging a diffracting lens for correcting longitudinal chromatic aberration in the violet area in the optical pickup device having interchangeability, arranging the diffracting lens in the optical path where a light flux emitted from a violet laser light source for high density optical disc and a light flux emitted from a laser light source for DVD and CD pass, rather than arranging the diffracting lens in the optical path where a light flux emitted from a violet laser light source for high density optical disc only passes, makes the structure of the optical pickup device to be simple, thus, it is possible to attain downsizing and cost reduction, because the number of parts can be reduced.
TOKKAI No. 2001-60336, for example, discloses an example of the optical pickup device which can conduct recording/reproducing of information properly for high density optical disc by correcting chromatic aberration in the violet area by arranging the diffracting lens in the optical path where a light flux emitted from a violet laser light source for high density optical disc and a light flux emitted from a laser light source for DVD and CD pass.
However, in the drawings and description in the aforesaid Patent Application, there is no disclosure about how to correct spherical aberration in the case of conducting recording/reproducing of information for high density optical disc. Spherical aberration generated in the optical pickup device grows greater in proportion to the fourth power of NA of an objective lens and to a reciprocal number of a wavelength. Therefore, for proper recording/reproducing of information for a high density optical disc, it is necessary to conduct correction of spherical aberration caused by errors of protective layer thickness of high density optical disc, focus jump between recording surfaces of a two-layer disc and by errors of a wavelength of a violet laser light source, by providing a means for correcting spherical aberration, in addition to the correction of the aforesaid longitudinal chromatic aberration. Namely, in the optical pickup device described in the aforementioned Patent Application, there is sometimes an occasion where recording/reproducing of information cannot be conducted properly for at least a high density optical disc among the high density optical disc, DVD and CD.